The present invention relates generally to gas turbines, for example, for electrical power generation and more particularly to the control of coolant flow to effectively cool the fillet region of the nozzle airfoils of the turbine.
Gas turbines typically include a compressor section, a combuster and a turbine section. The compressor section draws ambient air and compresses it. Fuel is added to the compressed air in the combustor and the air-fuel mixture is ignited. The resultant hot fluid enters the turbine section where energy is extracted by turbine blades, which are mounted to a rotatable shaft. The rotating shaft drives the compressor in the compressor section and drives, e.g., a generator for generating electricity or is used for other functions. The efficiency of energy transfer from the hot fluid to the turbine blades is improved by controlling the angle of the path of the gas onto the turbine blades using non-rotating airfoil shaped vanes or nozzles. These airfoils direct the flow of hot gas or fluid from a merely parallel flow to a generally circumferential flow onto the blades. Since the hot fluid is at very high temperatures when it comes into contact with the airfoil, the airfoil is necessarily subject to high temperatures for long periods of time. Thus, in conventional gas turbines, the airfoils are generally internally cooled, for example by directing a coolant through the airfoil.
Inside the airfoil, ribs are conventionally provided to extend between the convex and concave sides of the airfoil to provide mechanical support between the concave and convex sides of the airfoil. The ribs are needed to maintain the integrity of the nozzle and reduce ballooning stresses on the airfoil pressure and suction surfaces. The ballooning stresses are a result of pressure differences between the internal and external walls of the airfoil. The ribs define multiple cavities in the airfoil which define at least part of the coolant flow path(s) through the airfoil. The cavities may be cooled by impingement, using impingement inserts, or convection with or without turbulators on the ribs and/or airfoil walls. However, it is difficult to achieve the required cooling effectiveness in the airfoil to sidewall fillet regions at the exit end of the airfoil cavities. If the cavity is impingement cooled, the inserts cannot flare out to maintain the required impingement cooling gap due to insertability constraints. If this region is convectively cooled, due to the large flow area, the heat transfer coefficient are not sufficient to produce the required part life in this area. Therefore, previous designs using compressed air-cooling techniques would use film cooling to cool this region.
In advanced gas turbine designs, it has been recognized that the temperature of the hot gas flowing past the turbine components could be higher than the melting temperature of the metal. It has therefore been necessary to establish cooling schemes that more assuredly protect the hot gas components during operation. In this regard, steam has been demonstrated to be a preferred cooling media for gas turbine nozzles (stator vanes), particularly for combined-cycle plants. See for example, U.S. Pat. No. 5,253,976, the disclosure of which is incorporated herein by this reference. However, because steam has a higher heat capacity than the combustion gas, it is inefficient to allow the coolant steam to mix with the hot gas stream. Consequently, it is desirable to maintain cooling steam inside the hot gas path components in a closed circuit. Accordingly, in such a closed loop cooling system, film cooling of the fillet region is not permitted, so that effective cooling of this region remains problematic.
As noted above, significant backside cooling is required in turbine airfoils in the fillet region where the airfoil connects to the sidewall in order for the part to meet part life requirements. A design is required to achieve the desired cooling efficiency while minimizing the amount of cooling flow required. Also, downstream cooling of other areas on the airfoil sidewall must not be disturbed.
The present invention is embodied in a coolant flow control structure that channels cooling media flow to the fillet region. More particularly, the invention may be embodied in a flow control structure that defines a gap with the fillet region to achieve the required heat transfer coefficients in this region to meet the part life requirements.
Thus, in first aspect of the invention a flow control structure is provided for channeling cooling media flow to a fillet region defined at a transition between a wall of a nozzle vane and a wall of a nozzle segment, for cooling the fillet region, the flow control structure comprising: a base; and a main body, the main body being configured to define a crest generally at a transverse mid portion of the base and to define sloped walls from the crest toward longitudinal side edges of the base, thereby to define a gap with the fillet region to channel coolant flow along the fillet region.
According to another aspect of the invention, a turbine vane segment is provided for forming part of a nozzle stage of a turbine, the vane segment comprising: inner and outer walls spaced from one another; a turbine vane extending between the inner and outer walls and having leading and trailing edges, the vane including a plurality of discrete cavities between the leading and trailing edges and extending lengthwise of the vane for flowing a cooling medium through the vane; a plenum defined adjacent one of the inner and outer walls, at least one of the cavities of the vane being in flow communication with the plenum via an opening at a radial end of the vane to enable passage of cooling medium from the at least one cavity into the plenum; and a flow control structure for channeling cooling media flow to a fillet region defined at a transition between a wall of the vane and the one wall for cooling the fillet region.
According to yet a further aspect of the invention, a method of cooling the fillet region of a nozzle is provided that comprises: providing a nozzle vane segment including inner and outer walls spaced from one another; a turbine vane extending between the inner and outer walls and having leading and trailing edges, the vane including a plurality of discrete cavities between the leading and trailing edges and extending lengthwise of the vane for flowing a cooling medium through the vane; and a plenum defined adjacent one of the inner and outer walls, at least one of the cavities of the vane being in flow communication with the plenum via an opening at a radial end of the vane to enable passage of cooling medium from the at least one cavity into the plenum; disposing a flow control structure at the opening; flowing coolant medium through the cavity; channeling the flowing coolant medium at the outlet with the flow control structure to a fillet region defined at a transition between a wall of the vane and the one wall for cooling the fillet region.